Inductively coupled plasma source using induced eddy currents

ABSTRACT

Methods and apparatus are provided for generating an inductively coupled plasma using induced eddy currents. An inductively coupled plasma source of the invention generally comprises a body constructed substantially of a conductive material interrupted by at least one dielectric gap. Radio frequency power is coupled from a current carrier into the conductive body. The one or more dielectric interruptions in the conductive body are disposed so as to cause eddy currents to circulate about portions of the body and thereby couple RF power into a plasma in proximity to the conductive body. By utilizing induced eddy currents to couple power into a plasma, the invention allows for substantial bodies of conductive materials, such as structural metals, to be interposed between the induction coils that receive power from a power generator and the plasma.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to plasma processing sources, and moreparticularly to apparatus and methods for inductively coupled plasmaprocessing.

2. Brief Description of the Prior Art

Inductively coupled plasma sources in a variety of configurations areemployed in a broad range of industrial applications. Inductivelycoupled plasma processing chambers are used abundantly for modifying thesurface properties of materials, as for example in the manufacture ofmodern integrated circuits. Inductively coupled plasma sources may alsooperate as remote sources of activated gas species for downstreamprocessing operations, or as abatement devices for treatment of toxic orenvironmentally harmful materials.

In one form of well known inductively coupled plasma source, radiofrequency (RF) power is coupled from inductive coils into a plasmacontained within a dielectric enclosure. For example, the source maycomprise a cylindrical dielectric discharge tube wrapped by an inductivesource coil. When energized by an RF power generator, the sourceoperates like an air core transformer with the inductive coil as theprimary circuit and the plasma within the tube as the secondary circuit.Alternatively, induction coils may be disposed in a planar or conformalhelix configuration adjacent to a dielectric discharge chamber forcoupling of RF power into a plasma contained within the chamber.

The use of dielectric chamber materials to separate induction coils fromthe plasma discharge body can significantly limit the scale andoperational range of an inductively coupled plasma source. Structuraldielectric materials, such as quartz or sapphire, typically suffer frommechanical and thermal constraints when used in high power density andchemically reactive applications. The need to extract and dissipatethermal energy transferred from the plasma to the chamber walls is alsomore challenging when the chamber is constructed of dielectricmaterials. Cooling mechanisms such as forced air or circulating fluidsare not only complicated and expensive to implement, but also typicallyresult in reduced coupling efficiency of power to the plasma. Moreover,electrostatic coupling between the induction coils and the plasma canresult in localized ion bombardment of the chamber walls, which not onlyexacerbates the problem of chamber heat extraction but may over timeimpair the structural integrity of the chamber itself. Faraday shieldingcan be employed to decrease the capacitive coupling between the sourcecoils and the plasma, thereby reducing ion sputtering of the chamberwalls. A Faraday shield or cage employed for this purpose is typicallydesigned so as to suppress or minimize eddy currents within the shield.

In another form of inductively coupled plasma source, RF power iscoupled from inductive coils through a high permeability core materialto a ring discharge plasma. In this configuration, the source operatesas a magnetic core transformer with the ring plasma acting as asingle-turn secondary circuit. The ring plasma discharge may be confinedwithin a chamber of closed-path topology, such as a torus, as describedfor example in U.S. Pat. Nos. 3,500,118 and 4,431,898. The dischargechamber may be comprised of a dielectric material to ensure thatcurrents are coupled into the plasma rather than within the body of thechamber itself. The chamber may, however, be comprised substantially ofa conductive material provided that at least one insulating gap or breakis provided along the major circumference of the torus to prevent thechamber itself from acting as a short-circuited turn, as described forexample in U.S. Pat. No. 3,109,801. By permitting use of a nearlyall-metal chamber, which may be fluid cooled, issues of thermalmanagement are simplified. As a result, magnetic core inductivelycoupled plasma sources are useful for generating charged particles andchemically active species at relatively high densities and power levels.A topologically toroidal plasma source is a complex apparatus, however,that does not lend itself to simple design and manufacturing forcommercial applications. Moreover, the performance a toroidal source islimited by the quality, expense, and ability to cool the highpermeability ferrite materials that must typically be employed foroperation with RF power sources in medium to high frequency ranges.

It would be desirable to construct an inductively coupled plasma sourcehaving a relatively simple configuration, such as a discharge tube, butwithout the attendant disadvantages of a plasma tube or chamberconstructed substantially of dielectric materials. It would be furtherdesirable if the plasma source were not dependent for its operation uponexpensive ferrite transformer materials.

SUMMARY OF THE INVENTION

This invention provides methods and apparatus for creating aninductively coupled plasma using induced eddy currents. The inventiongenerally comprises a body constructed substantially of a conductivematerial interrupted by at least one dielectric break. Alternatingcurrent power is inductively coupled from a current carrier, such as aninduction coil, into the conductive body. The dielectric gap or gaps inthe conductive body are disposed so as to cause eddy currents tocirculate about portions of the conductive body and thereby couple RFpower into an adjacent plasma.

In one embodiment of the invention, a plasma chamber comprisesconductive segments aligned longitudinally to form a hollow tube, andseparated by dielectric breaks or gaps. An induction coil is disposedcoaxially about the outer perimeter of the chamber formed of theconductive segments. A power supply provides alternating current to theinduction coil, which creates alternating magnetic fields in the spaceoccupied by the chamber. Because of the dielectric separation betweenthe conductive chamber segments, the alternating magnetic fields induceeddy currents that circulate radially along the surfaces of theindividual segments, which are thick relative to the surface currentskin depth. Net alternating currents are thereby induced along theinterior conductive surfaces of the discharge tube. These net currentsin turn couple power into a plasma contained within the hollow interiorportion of the chamber.

By utilizing induced eddy currents to couple power into a plasma, theinvention allows for substantial bodies of conductive materials to beinterposed between the induction coils that receive power from a powergenerator and the plasma. Thus, in one embodiment of the invention, aninductively coupled plasma source may be constructed in the form of asimple linear or solenoidal discharge tube, but wherein the tube iscomposed almost entirely of a conductive material such as a metal. Theuse of a nearly all-metal plasma chamber can have many advantages,including simplified manufacturability and thermal management. A plasmachamber that is substantially conductive also largely avoids the problemof ion bombardment of the chamber walls by reducing or eliminatingcapacitive coupling between the induction coils and the plasma. As aresult, an inductively coupled plasma source of the invention hasenhanced performance and durability compared to sources that relysubstantially upon structural dielectric materials for confinement ofthe plasma.

In one embodiment of the invention, conductive chamber segments areseparated by air gaps. Depending upon the application, a dielectricwindow material may also be provided between chamber segments in orderto maintain vacuum integrity or to confine the plasma. In anotherembodiment, the conductive chamber segments are constructed so thatadjoining surfaces of the segments mate flush with each other. Aninsulating coating or treatment, such as an anodization layer, isapplied to the adjoining surfaces. Alternatively, a dielectric adhesiveor filler is disposed between the adjoining surfaces of the conductivesegments. When the conductive segments are assembled, the resultingdischarge chamber is a nearly seamless and unitary metallic article thathas embedded within it the dielectric breaks needed for formation ofinduced eddy currents within the chamber body.

In forming a plasma chamber of conductive segments, the dielectricbreaks between segments may extend along the entire length of thechamber. The chamber may also be formed by joining the segments at theirlongitudinal ends using caps or rings of dielectric material.Alternatively, the conductive segments may be joined at theirlongitudinal ends with a conductive material. Although this provides aleakage current path that reduces the power coupled from the inductioncoils into the plasma, power loss may be minimized by making the path ofthe leakage current substantially longer than that of the eddy currents.

Conductive chamber segments may be configured to form a plasma chamberhaving any cross-sectional shape, including circular or rectangular.Conductive segments may also be disposed in other configurations inaccordance with the present invention so as to couple RF energy into aplasma by means of eddy currents induced within the segments. In oneembodiment, a planar fixture comprised of radially disposed conductivesegments separated by dielectric gaps is provided between a helicalinduction coil and a plasma. In another embodiment, radially disposedconductive segments form a conformal dome between an induction coil anda plasma. In appropriate configurations, plasma chambers of theinvention are suitable for use in numerous plasma processingapplications including inline abatement, dissociation, or processing ofworking gases; remote production of activated gases for downstreamprocessing; plasma modification of surface properties of a workpiece;glass cleaning, etching, or coating; physical or chemical vapordeposition of materials upon a process substrate; etching, coating,stripping or ashing of a substrate surface, as in production ofintegrated circuit wafers or memory disks; and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an inductively coupled plasma source in accordancewith one embodiment of the invention.

FIG. 2 is an orthographic view of the plasma discharge tube of theembodiment depicted in FIG. 1.

FIG. 3 is a cross-sectional view of the plasma discharge tube of theembodiment depicted in FIG. 1.

FIGS. 4 a, 4 b, and 4 c illustrate the plasma discharge tube of afurther embodiment of the invention.

FIG. 5 illustrates an inductively coupled plasma source adapted for usein a chemical vapor deposition (CVD) application in accordance with afurther embodiment of the invention.

FIGS. 6 a and 6 b illustrate inductively coupled plasma chambers inaccordance with further embodiments of the invention.

FIG. 7 illustrates an inductively coupled plasma source having anexternal plasma discharge in accordance with another embodiment of theinvention.

FIG. 8 illustrates an alternative embodiment of the invention having anexternal plasma discharge.

DETAILED DESCRIPTION

FIG. 1 illustrates an inductively coupled plasma source 10 in accordancewith one embodiment of the invention. An RF power source 12 furnishesalternating current to induction coils 14 disposed coaxially about asubstantially metallic plasma discharge tube 16 containing a plasmawithin. As illustrated in the embodiment of FIG. 1, plasma dischargetube 16 is configured as a hollow cylinder open at both ends 18 to allowfor gas inlet and exhaust, as for example in an inline gas processingapplication. Alternatively, the plasma tube may be configured as asealed vacuum chamber having metered inlet and exhaust ports for feedand processing gases. Although not shown, the apparatus may alsocomprise impedance matching elements or circuitry disposed between RFpower source 12 and induction coils 14, as well as measurement andfeedback circuitry to regulate operation of the device. Also not shownare other features that may typically be included in a plasma processingsystem such as vacuum pumping manifolds, gas delivery connections ormanifolds, fluid cooling apparatus, plasma ignition electrodes or otherdevices, and mechanisms for workpiece mounting, transfer, or electricalbiasing.

FIGS. 2 and 3 represent orthographic and cross-sectional views,respectively, of the plasma discharge tube 16 of FIG. 1. In thisembodiment, plasma discharge tube 16 is formed of a metal cylinderhaving longitudinal grooves 22 through the body of the cylinder. Agastight dielectric seal comprising gas seal 24 and dielectric cover 26is disposed across each groove 22 in order to preserve the gasconfinement integrity of the discharge tube 16. The longitudinal grooves22 thus divide the walls of plasma discharge tube 16 into longitudinallyaligned conductive segments 28 interrupted by dielectric breaks.

Alternating current 32 applied to induction coils 14 causes time-varyingmagnetic fields to develop in the space occupied by the chamber 16.Conductive chamber segments 28 are of a thickness that is greater thanthe skin depth as determined by the material properties of the segments28 and the operating frequency of the RF power source 12. Eddy currents34 thus develop that circulate radially along the surfaces of eachconductive chamber segment 28. As a result, a virtual current loop 36 isestablished along the interior conductive surfaces of the chamber 16.The virtual current loop 36 further creates time-varying magnetic fieldsin the interior plasma containment portion of chamber 16, inducingcurrents within and thereby coupling power into the plasma 50.

Only one dielectric gap 22 need be provided in order to create the eddycurrents within the conductive chamber body needed to couple power intothe plasma within. In principle, the chamber may be comprised of anynumber of conductive segments 28 separated by dielectric gaps, providedthat the resulting segments are of sufficiently substantial dimension tocarry the required eddy currents and create the virtual current loop 36.The conductive segments 28 may be comprised of a common structural metalsuch as aluminum or stainless steel, or any other conductive materialsuitable to the thermal and chemical environments of a particular plasmaprocessing application. Preferably, each conductive segment 28 is alsosufficiently substantial to have embedded within it one or more coolingchannels 40 through which cooling fluids may circulate, while retainingsuch structural properties as may be required of the segment. Fittings42 may be provided for connection of the cooling channels 40 to a sourceof chilled water or other cooling fluid (not shown) for thermalmanagement of the plasma source apparatus.

Dielectric gaps 22 need only be of sufficient width and dielectricstrength to resist the peak-to-peak breakdown voltages that developacross conductive segments 28 upon application of RF power to theinduction coils 14. In the embodiment of FIG. 2, the dielectric gaps 22do not extend the entire length of the discharge tube 16. As a result, aleakage current path exists that reduces the power coupled from theinduction coils into the plasma. This power loss may be minimized to anacceptable level by making the discharge tube 16 substantially greaterin overall length than the region occupied by induction coils 14, thusmaking the path of the leakage current substantially longer than that ofthe eddy currents that couple power into the plasma. Alternatively, theleakage current may be reduced or eliminated by forming one or more ofthe dielectric gaps of a structural insulating material that extends thelength of the chamber, or by joining conductive segments at theirlongitudinal ends using caps or rings of a structural dielectricmaterial.

By transferring the RF power furnished to induction coils 14 into avirtual current loop within the plasma discharge tube, theelectromagnetic fields applied to the plasma are concentrated andcoupling of power to the plasma is improved. Due also to the enhanceddurability and thermal properties of a nearly all-metal plasma chamber,significantly greater power densities can be realized with a plasmasource of the invention as compared with a conventional discharge tubeapparatus of similar scale.

FIGS. 4 a, 4 b, and 4 c illustrate a plasma discharge tube in accordancewith another embodiment of the invention. Conductive discharge tubesegments 128 comprise mating surfaces 122 treated with an electricallyinsulating layer 124. The insulating layers 124 may be provided byanodization or similar treatment of the conductive surface, or byapplication of a dielectric coating material such as an epoxy adhesive.As shown in FIGS. 4 b and 4 c, conductive segments 128 assemble to forma hollow cylindrical discharge tube 120 having embedded longitudinaldielectric interruptions 126 and cooling channels 140. Mating surfaces122 may be made optically flat so that additional gas sealing betweensegments 128 is not required. Alternatively, gas sealing may beaccomplished through use of a dielectric filler or adhesive betweensegments, such a high temperature epoxy resin or refractory ceramicpaste.

When alternating current 132 is applied to induction coils 114, inducededdy currents 134 develop within conductive chamber segments 128 andcreate virtual current loop 136. The virtual current loop 136 inducescurrents within a plasma 150 contained within the hollow portion ofdischarge chamber 120.

FIG. 5 illustrates an embodiment of the invention adapted for use in achemical vapor deposition (CVD) application. Plasma chamber 516 is aconductive hollow body having one or more feed gas inlets 530 at one end518 of the body and a substantially open discharge region at opposingend 520. Also provided near the discharge end of plasma chamber areports 532 for one or more precursor gases 534 to be injected into theprocess zone. The cross-sectional aspect ratio of plasma chamber 516 isoptimized for dispersal of CVD reaction precursors in the vicinity of atranslating workpiece 536.

A plurality of longitudinal grooves 522 is provided through theconductive body of plasma chamber 516, creating a series oflongitudinally aligned conductive segments 528 separated by dielectricbreaks. If needed, dielectric covers and gas seals may be providedacross the grooves 522. Disposed about the chamber body are inductioncoils 514 oriented transversely to the conductive segments 528. Whenenergized by RF current, the induction coils induce eddy currents in theconductive segments, which in turn couple RF power into a plasma 550contained within the hollow plasma chamber 516. As an example, theplasma source of this embodiment may be used to generate a plasma froman oxygen feed gas injected at first gas inlets 530. A silane or othersilicon-bearing precursor is injected into the plasma 550 at secondinlets 532 where it dissociates and reacts to form a Si_(X)O_(Y)compound, such as SiO₂, which is deposited as a solid film upon thetranslating substrate 536.

In accordance with alternative embodiments of the invention, aninductively coupled plasma is generated by inducing eddy currents inconductive bodies that form only a portion of a plasma confinementchamber, or that are ancillary to the chamber. In FIG. 6 a, plasmaprocessing chamber 602 is an enclosed cylinder containing a workpiece(not shown). Disposed atop processing chamber 602 is a conductive disk604 having a plurality of radial grooves 606, creating an array ofradially disposed conductive segments 608. Adjacent to conductive disk604 are helical induction coils 610. When energized by RF current, theinduction coils 610 induce eddy currents in the conductive segments 608,which in turn couple RF power into a plasma contained within processingchamber 602 and that acts upon the workpiece. The same principle isillustrated in the embodiment of FIG. 6 b, wherein radially disposedconductive segments form a conformal dome between a helical inductioncoil and a plasma.

FIG. 7 illustrates an embodiment of the invention that generates anexternal inductively coupled plasma. A substantially conductive body isa hollow cylindrical tube that comprises longitudinally alignedconductive segments 728 interrupted by dielectric breaks 722. Disposedwithin the conductive body are induction coils 714 wound transversely tothe conductive segments 728. A flux concentrating magnetic material (notshown) such as a ferrite core may be disposed within induction coils 714to enhance magnetic fields generated by the coils. When energized by RFcurrent, induction coils 714 induce eddy currents 734 in the conductivesegments and create virtual current loop 736 external to the cylindricaltube. The virtual current loop 736 induces currents within a coaxialplasma 750 external to the cylindrical tube. Plasma 750 may be providedas an exposed external discharge, or alternatively may be confinedwithin an outer coaxial enclosure (not shown). If a confined plasma isto be subatmospheric, gastight dielectric windows 724 may also be addedto seal dielectric breaks 722.

An alternative embodiment of the invention that generates an externalinductively coupled plasma is illustrated in FIG. 8. Conductive body 820is disposed adjacent to a current carrier 814. In cross section,conductive body 820 is formed so as to have a conductive portion 828that surrounds a hollow cavity with a wall that is interrupted by adielectric air gap 822. When current carrier 814 is energized by RFcurrent, eddy currents 834 are induced in conductive portion 828 andcreate virtual current loop 836. The virtual current loop 836 inducescurrents within a plasma 850 in the hollow interior cavity of conductivebody 820. Due to the position of air gap 822, however, the plasma 850 isnot confined within conductive body 820 but may appear as an externaldischarge. In the embodiment of FIG. 8, conductive body 820 is disposedas a body of revolution about current carrier 814, resulting in coaxialring plasma discharge 850.

Although there is illustrated and described herein specific structureand details of operation, it is to be understood that these descriptionsare exemplary and that alternative embodiments and equivalents may bereadily made by those skilled in the art without departing from thespirit and the scope of this invention. Accordingly, the invention isintended to embrace all such alternatives and equivalents that fallwithin the spirit and scope of the appended claims.

1. A plasma source apparatus, comprising: a) a substantially conductivebody comprising one or more conductive segments interrupted by at leastone dielectric break; b) a current carrier adjacent to the substantiallyconductive body; and c) a power supply that furnishes alternatingcurrent power to the current carrier, the current carrier inducing eddycurrents within the one or more conductive segments, the eddy currentscoupling power into a plasma adjacent to the substantially conductivebody.
 2. The apparatus of claim 1 wherein the substantially conductivebody forms at least a portion of a plasma chamber that substantiallyconfines the plasma.
 3. The apparatus of claim 2 wherein the plasmachamber is substantially cylindrical and the at least one dielectricbreak comprises one or more grooves that separate at least a portion ofthe chamber into the conductive segments.
 4. The apparatus of claim 3wherein the conductive segments are longitudinally aligned, and whereinthe current carrier is an induction coil disposed coaxially about theplasma chamber.
 5. The apparatus of claim 3 wherein the one or moregrooves are covered by gastight dielectric seals.
 6. The apparatus ofclaim 2 wherein the plasma chamber is a substantially cylindrical bodyformed by longitudinal alignment of the conductive segments, and whereinthe current carrier is an induction coil disposed coaxially about theplasma chamber.
 7. The apparatus of claim 6 wherein the at least onedielectric break comprises an insulating layer disposed upon matingsurfaces of the conductive segments.
 8. The apparatus of claim 7 whereinthe insulating layer results from an anodization treatment of one ormore of the mating surfaces.
 9. The apparatus of claim 7 wherein theinsulating layer comprises a dielectric adhesive.
 10. The apparatus ofclaim 1 wherein one or more cooling channels is disposed within at leastone of the conductive segments.
 11. The apparatus of claim 1 wherein thecurrent carrier is disposed within a hollow region of the substantiallyconductive body.
 12. The apparatus of claim 1, further comprising atleast one inlet for a gas to enter the plasma chamber.
 13. The apparatusof claim 1 wherein the dielectric break interrupts a wall of a cavity inat least one of the one or more conductive segments.
 14. A plasmaprocessing system, comprising: a) a substantially conductive bodycomprising one or more conductive segments interrupted by at least onedielectric break; b) a current carrier adjacent to the substantiallyconductive body; and c) a power supply that furnishes alternatingcurrent power to the current carrier, the current carrier inducing eddycurrents within the one or more conductive segments, the eddy currentscoupling power into a plasma adjacent to the substantially conductivebody.
 15. The system of claim 14, further comprising a plasma chamberthat substantially contains the plasma.
 16. The system of claim 15wherein the substantially conductive body forms at least one portion ofthe plasma chamber.
 17. The system of claim 16 wherein the plasmachamber is a substantially cylindrical body formed by longitudinalalignment of the conductive segments, and wherein the current carrier isan induction coil disposed coaxially about the plasma chamber.
 18. Thesystem of claim 16 wherein the substantially conductive body is a planardisk formed by radial dispersal of the conductive segments, and whereinthe current carrier is a helical induction coil disposed adjacent to theplanar disk.
 19. The system of claim 16 wherein the substantiallyconductive body is a conformal dome formed by radial dispersal of theconductive segments, and wherein the current carrier is a helicalinduction coil disposed about the conformal dome.
 20. The system ofclaim 15, further comprising a first gas inlet for injection of aprocessing gas into the plasma chamber.
 21. The system of claim 20,further comprising a second gas inlet for injection of a precursor gasinto the plasma.
 22. A method of plasma processing, comprising: a)providing a substantially conductive body comprised of one or moreconductive segments interrupted by at least one dielectric break; b)inducing eddy currents in the conductive segments by furnishingalternating current power to a current carrier disposed adjacent to thesubstantially conductive body; and c) coupling power into a plasmaadjacent to the substantially conductive body using the induced eddycurrents.
 23. The method of claim 22 wherein the plasma is substantiallycontained within a plasma chamber.
 24. The method of claim 23 whereinthe substantially conductive body forms at least one portion of theplasma chamber.
 25. The method of claim 22, further comprising the stepof dissociating a feed gas in the plasma.
 26. The method of claim 22,further comprising the step of abating a feed gas in the plasma.
 27. Themethod of claim 22, further comprising the step of etching material froma workpiece using the plasma.
 28. The method of claim 22, furthercomprising the step of depositing material from the plasma upon aworkpiece.
 29. The method of claim 23, further comprising the steps ofinjecting a feed gas into the plasma chamber to form an activated gas;injecting a precursor gas into the plasma, the precursor gas reactingwith the activated gas to form a vapor deposition compound; anddepositing the vapor deposition compound upon a workpiece.